Conventional turbomolecular vacuum pumps include a housing having an inlet port, an interior chamber containing a plurality of axial flow pumping stages and an exhaust port. The exhaust port is typically attached to a roughing vacuum pump. Each axial flow pumping stage includes a stator having inclined blades and a rotor having inclined blades. The rotor and stator blades are inclined in opposite directions. The rotor blades are rotated at high speed to provide pumping of gases between the inlet port and the exhaust port. A typical turbomolecular vacuum pump may include nine to twelve axial flow pumping stages.
Variations of the conventional turbomolecular vacuum pump are known in the prior art. In one prior art configuration, one or more of the axial flow pumping stages are replaced with disks which rotate at high speed and which function as molecular drag stages. This configuration is disclosed in U.S. Pat. No. 5,238,362 issued Aug. 24, 1993 to Casaro et al. A turbomolecular vacuum pump including an axial turbomolecular compressor and a molecular drag compressor in a common housing is sold by Varian Associates, Inc. under Model No. 969-9007. Turbomolecular vacuum pumps utilizing molecular drag disks and regenerative impellers are disclosed in German Patent No. 3,919,529 published Jan. 18, 1990.
Molecular drag compressors include a rotating disk and a stator. The stator defines a tangential flow channel, and an inlet and an outlet for the tangential flow channel. A stationary baffle, often called a stripper, disposed in the tangential flow channel separates the inlet and the outlet. As is known in the art, the momentum of the rotating disk is transferred to gas molecules within the tangential flow channel, thereby directing the molecules toward the outlet and pumping the gas.
Some instruments and processing systems have two or more vacuum chambers which it is desirable to operate at different pressure levels. The chambers may be connected through one or more orifices that are small enough to permit establishment of different pressure levels. Examples include mass spectrometers, molecular beam systems and ion beam systems. One approach is to connect a separate vacuum pump to each of the vacuum chambers. Another approach, which is typically more economical, is to utilize a single vacuum pump having two or more inlets which are connected to different points in a single vacuum pump. The inlets are connected to different vacuum chambers.
An example of a prior art dual inlet turbomolecular vacuum pump 10 is shown in FIG. 4. The turbomolecular vacuum pump (turbopump) 10 includes a first pumping section 12, a second pumping section 14 and an interstage region 16 between pumping sections 12 and 14. First pumping section 12 includes axial flow pumping stages 20, 22, etc., and second pumping section 14 includes axial flow pumping stages 30, 32, etc. A housing 40 has a first inlet port 42 coupled to an inlet of first vacuum pumping section 12, a second inlet port 44 coupled through a conduit 46 to interstage region 16, and an exhaust port 48 coupled to an outlet 50 of second vacuum pumping section 14. Each of the axial pumping stages 20, 22, 30, 32, etc. includes a stator having inclined blades and a rotor having inclined blades. The rotor of each axial pumping stage is connected by a shaft 52 to a motor 54.
In use, first inlet port 42 is connected to a first vacuum chamber (not shown) at a relatively low pressure and second inlet port is connected to a second vacuum chamber (not shown) at a higher pressure level. The first and second chambers are evacuated simultaneously by turbopump 10.
The turbopump configuration shown in FIG. 4 provides generally satisfactory performance, but has certain disadvantages. The interstage region 16 has a relatively large axial dimension parallel to shaft 52 in order to provide adequate gas conductance between second inlet port 44 and second pumping section 14. This requires a lengthening of shaft 52 in order to provide the same performance as an equivalent single inlet turbopump. This results in increased size and cost of the turbopump. In addition, since the shaft and rotors are typically cantilevered from the motor end of the turbopump, the increased shaft length may give rise to problems in balancing the turbopump for high speed operation and in reduction of bearing life.
Accordingly, it is desirable to provide vacuum pump configurations which overcome one or more of the above disadvantages.